IE9/10 is the default browser on Windows Phone 7, 7.5, 8. Windows 8, Windows Surface RT, Windows Surface Pro.
That said, we know the performance of IE is, sub par. WebGL and sound drivers are not implemented. Overall compared performance is lacking. The only way to fix this is to make our voices heard. A good place to start is the feedback forum.
I'm still trying to figure out how to register and submit feedback though. I will update this as I learn more.
I'm following this thread, to get better instructions.